The present invention relates generally to tissue removal techniques and, more particularly, to generating locations for scanning with a laser to achieve a desired ablation profile for correction of errors in vision during laser eye surgery.
A scanning system has the ability to trace out an arbitrary pattern with a small low energy spot. In most cases, a small spot equates to finer scanning details but at the expense of requiring more pulses to remove a given volume. The notion that a small spot will give a better fit than a large spot is generally true for arbitrary spot and ablation shapes, but the spot shapes can also affect the fit. For instance, trying to fit round disks into a square shape will result in a residue. The emphasis on getting a good fit should be on choosing a good balance of spot geometry and size.
Small spot scanners have their problems. A small spot will have smaller coverage per shot, thereby requiring more pulses to remove a given ablation volume (inversely proportion to the area of the spot size). A larger spot will require a substantially smaller number of pulses but at the expense of resolution. Understanding how treatment varies with spot overlap will ease our ability to create ablation patterns.
Present ablation algorithms follow a removal or subtractive process. An ablation spot is placed on a location along the two-dimensional corneal surface. This spot ablates a volume of tissue from the surface to produce a crater on this surface. Another spot is applied at another location along the two-dimensional corneal surface and another crater is produced. The challenge lies in placing the craters in the correct locations such that adding up the overlapping craters will produce the desired surface without producing undesirable residues and requiring excessive processing time.